Method of making galvannealed ferrous metal of improved solderability



nited States 3,540,943 METHOD OF MAKING GALVANNEALED FER- ROUS METAL F IMPROVED SOLDERABILITY Edward M. Grogan, Pittsburgh, Pa., assignor to United States Steel Corporation, a corporation of Delaware N 0 Drawing. Continuation-impart of application Ser. No. 391,010, Aug. 20, 1964. This application Aug. 8, 1968, Ser. No. 751,023

Int. Cl. C23f 7/00 U.S. Cl. 148-614 6 Claims ABSTRACT OF THE DISCLOSURE This application, which is a continuation-in-part of my co-pending application, Ser. No. 391,010, filed Aug. 20, 1964, now abandoned, relates to a method of improving the solderability of galvannealed ferrous metal. The terms ferrous and ferrous metal as used herein refer to iron and iron alloys.

Galvannealed ferrous metal articles such as sheets or strip are produced by passing the material after galvanizing, i.e. zinc coating, through a heated chamber that maintains the coating metal molten until it alloys substantially completely with the ferrous base. Galvannealed products possess a silvery matte finish free of spangles and are excellent paint bases.

It is presently believed that the presence of aluminum in the zinc, i.e. spelter, used in galvanizing prior to annealing is responsible for the difficulty in soldering commercial galvannealed ferrous metal. Aluminum oxidizes readily and the oxide film, which may be a mixture including aluminum oxide, appearing at the coating surface is nearly impervious to solder fluxes and makes solder operations extremely difiicult. Normally, the spelter may contain from about 0.007 to 0.2% aluminum. Although improved solderability could be attained by using aluminum-free zinc coatings, this approach is not commercially practical because a small amount of aluminum is desirable in galvanized coatings and the same production lines are used in galvannealing as are employed to produce galvanized material.

There are several methods of removing the undesirable oxide film appearing on galvannealed metal surfaces but most are objectionable because they may interfere with paint adhesion or are diflicult to control. One such method involves treating the surface with mineral acids to dissolve the oxide. However, this technique tends to remove too much coating along with the oxide and further, requires extensive rinsing.

The present invention provides a convenient and effective method of improving the solderability of galvannealed ferrous metal. According to the invention, galvannealed ferrous metal of improved solderability is made by galvanizing a ferrous metal base to provide a coating of zinc thereon, which also contains a small amount of aluminum, galvannealing the aluminum-containing, zinc coated base by heating to substantially completely alloy the coatiug with the ferrous metal.

An oxide film forms at the coating surface which is atent O disrupted by contacting, e.g. by dipping, spraying, etc., the galvannealed ferrous metal with an aqueous solution of a compound selected from the group consisting of sodium fluoborate, ammonium fluoborate, sodium hydroxide and potassium hydroxide. Of these compounds the fluoborates are much preferred for reasons which will appear later. Short contact times are effective and the process can be adapted to continuous galvannealing systems. Although the fiuoborate salts are acid, there is almost no evolution of hydrogen and consequently no loss of coating even at elevated temperatures.

The precise explanation of how the solution functions to improve the solderability is not fully understood. However, When using fluoborate solutions it is believed that the acidic action of the fluoborate solution first dissolves the surface oxide. As the fiuoborate reacts with the metallic coating, a complex zinc salt is formed which is practically insoluble in water and even in dilute fluoboric acid and tends to stop the reaction. The zinc salt, which is thought to have the formula Zn(BF is believed to form as a thin, invisible film on the surface. It has been suggested that this film also prevents further air oxidization of the surface and helps the solder flux to remove oxide film that may form subsequently. In any case, it has been found that the galvannealed article after treatment with one of the solutions described above, can be soldered with comparatively mild organic-type fluxes. Moreover, the solder-flow characteristics of galvannealed sheet treated with this process are equal to that of those of untreated continuous galvanized material and are vastly superior to untreated galvannealed material.

solderability can be evaluated by means of a test that measures the solder flow of a standard size solder pellet. As the solder spot is melted on a hotplate, e.g. at 525 F., the pellet Will either fiow or bead depending upon the flow properties of the sheet surface. Good flow properties will cause the melted solder pellet to spread over a large area with the result that the thickness of the solder spot (measured with a micrometer) is reduced accordingly. Hence low solder spot thicknesses indicate good solderability. Comparative solder performance of samples which have been treated according to my invention and an untreated sample is tabulated below and illustrates the improvement obtainable by practicing the invention.

The solder flow characteristics of the fluoborate treated samples are even superior to that of the continuous galvanized product. The data given in Table I are averages of approximately 20 tests using various organic and acidtype fluxes.

Galvannealed ferrous articles treated according to the invention may be water rinsed following the fluoborate treatment to improve paint adhesion. Also, a water rinse may be preferred if a bright surface finish is desired, since the fiuoborate treatment produces a slight darkening of the surface. However, paint tests have shown that samples given the fiuoborate treatment according to the invention and without rinsing have paint adhesion almost as good as that of the water rinsed samples. However, when using a solution of potassium or sodium hydroxide, the treated article must be washed if it is to be painted. The galvannealed article should not be rinsed if it is to be treated with a corrosive inhibitor such as sodium dichromate. Corrosion inhibitors may be added directly to the fluoborate solution for application to the galvannealed material. However, inhibitors cannot be added to the hydroxide solutions.

The following examples are submitted to illustrate various practices of the invention.

Samples (4 inches by 8 inches) of galvannealed sheet were immersed in aqueous solutions ofsodium fluoborate or ammonium fluoborate as indicated in Table II below. A control sample was treated in distilled water. The temperature of the solutions was maintained the same and the duration of the treatment used for each sample is also given in the table. After contact with the fluoborate solutions, the samples were rinsed with water, air dried after removal of excess water, and tested for solderability. The results of the solderability tests (solder-spot thickness test) given in the table represent an average of siX trials in each case.

1% NaBF Dist. Water.--

Results of various other tests are shown in Table III below. Each solder-spot thickness represents an average of at least six tests.

TABLE III Solderspot thick- Treating ness, time, secs. mils Solution:

10% NH BFi 5 7. 2 10% NH4BF4---- 15 8.1 10% NH4BF4.- 60 10.0 6% NH4BF4 5 9. l 6% NH4BF4 15 6. 5 6% NH4BF4 60 8. 9 3% NH4BF4--- 5 6. 8 3% NH4BF4- 15 9. 9 3% NgiBgiifl. 6g

15 7. 1% NILBF 60 7. 2 2% NaO 20. 3 2% NaOH 15 10. 6 2% NaOH 6O 3. 4 3% NaOH 5 15.1 3% NaO 15 7. l 5% NaOH 30 9.7 10% M13154. 5 4. s 10% NaBlh. 5. 9 6% NaBF4 5 7. 1 6% NaBF4 15 5. 1 5% NaBFA- 30 5.1 2% Nauru-.- 5 9.1 2% NaBF4. 15 3.1 2% NaBF4 60 4.6 3% NaBF 5 4.8 3% NaBFi. 15 5. 3 1% NaBF 5 7. 8 1% NaBF4 15 5.8 2% KO 5 40. 5 2% KO 15 16. 5 2% KO 50 7.9 5% KO 30 5. 3

It has been found that even very brief contact times, e.g. one or two seconds produces satisfactory fluoborate coatings and will improve solderability of the galvannealed article. Higher temperatures also speed the coating process. Longer times and/or greater concentrations are required when using the hydroxides. The preferred conditions for the practice of the invention include a contact time of at least two seconds and temperatures in the range of to 200 F. Various concentrations of the solutions may be used and solutions containing one to ten weight percent of the compound have been found to be satisfactory. Stronger solutions may be used but little additional advantage will be gained. It is also advantageous, particularly in a continuous processing system, to cool the galvannealed ferrous metal to below about 500 F. before contacting with the aqueous fluoborate solution to minimize distortion and warping of the article by a drastic temperature reduction.

The fluoborate treatment has the additional advanage of removing smudges that sometimes occur during galvanizing and the sheet has a cleaner, more attractive appearance.

To summarize, the invention requires that the galvannealed surface be contacted with an aqueous solution of a compound of the group consisting of sodium fluoborate, ammonium fluoborate, potassium hydroxide and sodium hydroxide. Obviously to he an aqueous solution of the compound, the compound must be the principal ingredient added to the water. However, lesser amounts of other materials having slight or no adverse effect on the solderability characteristic of the coating may be present. For example, a corrosion inhibitor may be added to the fluoborate solutions. The fluoborate solutions are preferred to the hydroxide solutions because better results are obtained at the same concentrations and treating times. Thus, the fluoborate solutions may be used in a continuous process with the galvannealing while using a relatively weak solution, whereas the hydroxide solutions would not be practical in the continuous process. In addition, more additives may be used with the fiuoborates and washing of the product is required for fewer end uses of the coated product. For example, to lower the pH value of fluoborate solutions, fluoborate acid may be added thereto and to raise the pH value of fluoborate solutions, ammonium or sodium hydroxide or sodium carbonate may be added thereto. Mixtures of sodium fluoborate and ammonium fluoborate may also be used at various pH levels.

While several embodiments of my invention have been described, it will be apparent that other adaptations and modifications may be made without departing from the scope of the following claims.

I claim:

1. A process for producing galvannealed ferrous metal of improved solderability which comprises providing a ferrous metal base having thereon a coating of Zinc metal containing aluminum with the coating completely alloyed with said ferrous base, contacting said galvannealed ferrous metal for at least 2 seconds with an aqueous solution consisting essentially of water, at least 1% by weight of a substance of the group consisting of sodium fluorborate, ammonium fluoborate, and mixtures of sodium fluoborate and amminium'fluoborate, and lesser amounts of other ingredients commonly present in coating solutions.

2. A continuous process according to claim 1 in which immediately preceding the contacting step, a ferrous metal base is galvanized with zinc metal containing aluminum to provide a coating thereon, and said coated base is galvannealed by heating to substantially completely alloy said coating with said ferrous metal.

3. A process according to claim 1 wherein said aqueous solution has a concentration of 1 to 10% by weight of said substance.

4. A process according to claim 3 wherein said galvannealed ferrous metal is contacted with said aqueous solu tion at a temperature in the range of about 130 to 200 F.

5. A process according to claim 2 wherein said aqueous solution has a concentration of 1 to 10% by weight of said substance.

6. A process according to claim 5 wherein said galvan nealed ferrous metal is contacted with said aqueous solution at a temperature in the range of about 130 to 200 F.

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227.142 3/1960 Australia. 29488;1342 

